This invention, concerned with nuclear reactors, is to introduce at the lower head of the reactor vessel a gap forming structure by which a coolable geometry is maintained to prevent overheating and ultimately failing of the lower head wall due to accumulated molten core debris in the event of a core meltdown accident. The lower head herein refers to a concave paraboloidal or cylindrical vessel that comprises as the reactor fluid boundary. Should such a severe accident occur, the core material inside the reactor vessel may melt due to overheating and then relocate down to the vessel lower head. In this event, direct contact by the vessel lower head with molten core material may heat up and deform the metallic reactor vessel lower head causing it to rupture, thereby posing a risk of massive release of radioactive materials. The structure of this invention creates an engineered gap which prevents the molten core material from directly contacting the vessel inner surface such that: firstly, it prevents rapid heating of the reactor vessel lower head; secondly, it helps achieve a defense against severe accidents by the secured water cooling effect inside the gap and hence prevents the reactor vessel lower head from failing.
Conventional water cooled reactors do not have gap structures for protection against severe meltdown accidents. Therefore, should a severe accident occur and the molten core material relocate downward and accumulate on the vessel lower head, a minimal gap may naturally form that may possibly be too small and irregular for sufficient cooling of the debris depending on the extent of the accident. In the Three Mile Island Unit-2 (TMI-2) accident of 1979, which turned out to be a severe accident involving core meltdown, direct contact of the vessel lower head with molten core debris resulted in excessive heating of the vessel inne wall to near its melting temperature. Yet, for some unexplained mechanisms, the vessel was subsequently, cooled to avoid vessel failure. To explain the rapid cooling, it is theorized that limited water cooling was achieved inside a gap formed between the molten core debris and the inner surface of the lower head, but its non-uniform and irregular configurations led to developing hot spots. In the TMI-2 accident, there remained insufficient margin separating the failure of the reactor vessel lower head despite the fact that the amount of relocated core debris of nineteen tons was not more than one-fifth of the core materials. Therefore, if a severe accident were to occur with a greater extent of melting, the cooling of the reactor vessel lower head may be inadequate with only a natural cooling mechanism and may potentially result in reactor vessel rupture followed by ejection of the molten core debris. Consequently, a large mass of high temperature materials may be released out of the vessel to trigger further chemical and thermal reactions with the structures in the containment building. This may develop into an even more severe accident by threatening the integrity of the containment by way of high pressure and temperature.
An ex-vessel catcher was proposed by M. J. Driscoll and F. L. Bowman (M. I. T.) [U.S. Pat. No. 4,113,560]. In the ex-vessel catcher, the molten core debris solidifies with ex-vessel structures (graphite, sand etc.). The ex-vessel catcher can confine and isolate molten core debris within itself to block further reaction with the containment structures. It is intended to avoid the generation of heat and gases from the reaction of the molten core debris with containment floor concrete and prevent the massive escape of radioactive molten core debris through the bottom concrete. Nevertheless this particular measure only is designed to take effect after the rupture of a reactor vessel, and much of the heat and radioactivity may nonetheless be released into the containment building, and hence, additional safety systems are required for containment building cooling and protection. In the case of a liquid metal cooled fast breeder reactor, reactor under-core catcher structures have been used. But in the Fermi-I case, a core meltdown accident was provoked by the structure when it loosened due to a design inadequacy and blocked the coolant flow. In addition, the horizontal under-core catcher design of Fermi-I and of the SUPERPHENIX will not be effective for water cooled reactors due to deteriorated cooling with boiling and bubble stagnation underneath the plate.
An ex-vessel cooling has been proposed and demonstrated for its capability to externally cool the lower head (T. G. Theofanos, C. Liu, S. Addition, S. Angelini, O. Kymaelaeinen, and T. Samassi, "In-vessel Coolability and Retention of a Core-melt," DOE/ID-10460, November 1994). The method, while it is considered to be feasible for some limited number of plants in operation or under design, has the disadvantage in that it will take significant amount of time and water resources so as to flood large volume of the reactor cavity. In addition, overflooding or untimely submergence of the vessel may impose the risk of thermal shock of the irradiation embrittled vessel beltline areas.